In FIG. 21 a prior art MOSFET 100 is shown as known from U.S. Pat. No. 7,074,643 B2. The prior art MOSFET 100 is made of an n+ silicon carbide (SiC) substrate 80 and comprises between a first main side 20 and a second main side 22 an n− doped drift layer 2. On the first main side 20, two n++ doped source regions 3, 3′ are arranged, each which is separated from the drift layer 2 in lateral direction (i.e. in a direction parallel to the first main side 20) by a p doped channel layer 4, 4′ and on a side opposite to the first main side 20 by a p+ well layer 5, 5′, which is higher doped than the channel layer 4, 4′. In between such two source regions 3, 3′ surrounded by a channel layer 4, 4′ and a well layer 5, 5′, a p++ doped contact layer 65 is arranged, which laterally extends to the source regions. Due to its high doping concentration the p++ doped contact layer provides a good ohmic contact to a first main electrode 9 (source electrode). The contact layer 65 is a shallow layer spatially (i.e. in depth direction, which direction is vertical to the first main side 20) extending to a contact layer depth 67, which is less deep than the well layers 5, 5′, but electrically and mechanically contacting the well layers 5, 5′ in order to connect the well layers 5, 5′ to the source electrode 9. The contact layer 65 overlaps with the source regions 3, 3′ the channel layers 4, 4′, so that the contact layer 65 is the only p doped layer in contact with the first main electrode 9.
A similar prior art device is known from US 2010/200931 A1, which document shows in a SiC MOSFET with a p well structure and a p+ plug. There is a triple point between the n source regions, well layer and plug, such that the plug does not overlap with the n source regions and again, the plug is the only p doped layer in contact with the first main electrode.
US 2012/205670 A1 discloses a SiC MISFET, which has a highly p doped plug arranged in a greater depth than the source regions, but embedded in the p channel layer. Again, the emitter electrode contacts only the source regions and the highly doped plug.